Here's why:
* Snails are food: Goldfish are omnivores and will happily eat snails, especially smaller ones.
* Territoriality: Goldfish can be territorial, especially if they are in a small tank. They may chase or nip at snails, even if they don't intend to eat them.
* Different needs: Snails and goldfish have different needs in terms of water temperature, water parameters, and food.
However, there are some ways to make them compatible:
* Choose large snails: Snails like Nerite and Mystery snails are too large for most goldfish to swallow.
* Provide plenty of hiding places: This will give snails a safe place to retreat from the goldfish.
* Offer a varied diet: Feed the goldfish a high-quality diet to reduce their urge to eat snails.
* Keep a close eye on them: Observe the interactions between the snails and goldfish, and be prepared to remove snails if they are being harassed or eaten.
It's important to remember that every fish and snail is different, and some individuals may be more tolerant of each other than others.
If you're considering keeping snails and goldfish together, it's important to do your research and choose a tank size and species that are appropriate for both.